Micro gear system operating according to the strain wave gear principle with hollow shafts

ABSTRACT

A micro gear system, operating according to the principle of a strain wave gear system, a Wolfrom gear system, or a similar planetary gear system comprises at least one sun wheel ( 14 ) and at least two planet wheels ( 16 ) meshing with the sun wheel ( 14 ). The sun wheel ( 14 ) is rotatably fixed to drive shaft ( 20 ) placed along a longitudinal axis ( 18 ) of the gear system. The planet wheels ( 16 ) indirectly or directly mesh with a toothing which is rotatably fixed to a drive shaft ( 28 ) placed centrally along the longitudinal axis of the gear system. The drive shafts ( 20,28 ) are embodied as hollow shafts ( 30,32 ) having continuous bore holes ( 34,36 ) such that information or fluid media can be directed through the gear system. A sealing system ( 42 ) comprising a labyrinth seal ( 44 ) which includes a plurality of annular grooves ( 46 ) is disposed between the hollow shafts ( 30,32 ).

FIELD OF THE INVENTION

The invention relates to a gear system in particular a micro gearsystem, preferably on the principle of the strain wave gear system orWolfrom gear system or similar revolving-wheel gear systems of flatconstruction, which has a planetary gear system, comprising a sun wheeland at least two or more planet wheels meshing with the sun wheel, inwhich the sun wheel is seated in a manner fixed against relativerotation on a drive shaft disposed centrally with respect to alongitudinal axis of the gear system or is a component of this driveshaft, and the planet wheels mesh indirectly or directly with a gearwheel which is connected in a manner fixed against relative rotation toa driven shaft, which is disposed centrally with respect to thelongitudinal axis of the gear system.

BACKGROUND OF THE INVENTION

Such gear systems are already known from the prior art, for instanceProceedings of Actuator 2000, June 19-21, Bremen, “Pancake shaped microgear system with high transmission ratio” by R. Degen, W. Ehrfeld, andF. Michel; Das Micro Harmonic Drive der Micromotion GmbH: EinPräzisionsmikrogetriebe für Positionieraufgaben [Micromotion GmbH'sMicro Harmonic Drive: A Precision Micro Gear for Positioning Tasks] byR. Degen and F. Michel, F&M 109 (2001), No. 6, 48-50; and Proceedings ofInnovative Klein-und Mikroantriebe [Innovative Miniature and Micro DriveMechanisms], May 15-16, 2001, Mainz, “Micro Harmonic-Drive: InnovativeAntriebstechnik miniaturisiert mit LIGA” [Micro Harmonic Drive:Innovative Drive Technology Miniaturized with LIGA], by R. Degen.Examples of such known micro gear systems on the principle of strainwave gear systems, such as Harmonic Drive (registered trademark), arealso shown in FIGS. 2 a, b and FIG. 3 and will be described brieflybelow with respect to the principle of their mode of operation. It willalso be noted that the aforementioned published prior art is expresslyincorporated by reference into the disclosure content of the presentpatent application.

The gear principle of a strain wave gear system 56 is distinguished fromother designs, such as stationary spur gear systems and planetary gearsystems, by its precise and backlash-free transmission properties. Thesespecial properties have long been documented in robotics, machine tooldesign, measuring instruments, air and space travel, and medicaltechnology by the Harmonic Drive principle. Depending on theirembodiment, Harmonic Drive gear systems can be classified as either cupgear systems (FIG. 2 a) or flat gear systems (FIG. 2 b). The flatconstruction of this gear principle offers major advantages, especiallywith a view to micro drive systems, such as the low number of componentsrequired, a compact design, and the level of transmission ratio that isrequired for micro motors and that can be attained with a single gearstage. The basic elements of the Harmonic Drive gear system of flatconstruction are composed of the wave generator 12 and the gear wheelsknown as the flexible spline 22, circular spline 24 and dynamic spline26. As can be seen from FIG. 3, the wave generator 12 in Harmonic Drivegear systems of flat construction is preferably formed by a planetarygear system, comprising one central sun wheel 14 and two or more planetwheels 16 which mesh with the teeth of the sun wheel 14 and in turn meshwith the internal teeth of the flexible spline 22.

The fundamental principle of a Harmonic Drive gear system is that thewave generator 12 deforms the flexible spline 22 elliptically outward inat least two or more regions. As a result, the flexible spline 22 of thegear system, in the two outward-deformed regions, have tooth engagementssimultaneously with the two ring gears, that is, the circular spline 24and the dynamic spline 26. In the regions of the short semiaxis of theelliptically deformed flexible spline, the flexible spline is not intooth engagement with the circular spline 24 or the dynamic spline 26.

Upon rotation of the sun wheel 14 of the wave generator 12, the toothengagement regions of the flexible spline 22 migrate with the angularposition of the two planet wheels 16 of the wave generator, as shown inthe different angular views in FIG. 3. The relative motion between theflexible spline 22 and the circular spline 24 results from thedifference, for instance of two teeth, between the number of teeth ofthe flexible spline and of the circular spline. Upon one full revolutionof the planet wheels 16 of the wave generator, the flexible spline 22rotates relative to the circular spline 24 by the difference in thenumber of teeth between these two gear wheels. In the flat construction,the dynamic spline 26 serves as a driven element and has the same numberof teeth as the flexible spline 22 and therefore also has the same speedand direction of rotation.

The use of a planetary gear system as the wave generator 12 offers theadvantages, with a view to the degree of miniaturization of the microgear system, that all the gear components can be produced with highprecision using the known LIGA method described herein; the effort ofassembly is reduced, since the wave generator comprises only threeelements; the total transmission ratio of the gear is increased by theplanetary gear system, so that by means of such a gear system, in asingle stage, the very high speeds of rotation of micro motors can beadapted flexibly to individual requirements; and this variant of thewave generator 12 has very low moment of inertia, making highly dynamicpositioning operations possible.

SUMMARY OF THE INVENTION

With gear systems and micro gear systems of this kind as described atthe outset, it is the object of the invention to carry information or tofeed media through the gear system.

With the gear system having the characteristics recited at the outset asthe point of departure, this object is attained essentially in that thedrive shaft and driven shaft are each embodied as a hollow shaft with acontinuous bore or continuous conduit.

Because of these characteristics according to the invention, it ispossible to pass lasers, optical fibers, shafts, supply lines or thelike, or to feed media, through the gear system along the central axisof rotation, that is, the longitudinal axis of the gear system. Forinstance, it is possible to pass a vacuum for grasping tasks or handlingpurposes centrally through the gear system. It is equally possible foroptical sensors to look through the gear system and detect objects onthe opposite side of the gear system. In conventional multi-stage gearsystems or micro gear systems this is as a rule not possible, since thecentral bore would have to extend simultaneously through a plurality ofsuccessively connected gear stages, whose gear wheels or parts generallydo not have a common central axis of rotation. In the micro gear systemof the invention, despite the very high transmission ratio in the rangeof ca. 100 to 1500, with a single gear stage there is one central axisof rotation for the gear system, so that through the bore of the driveshaft or through a bore of the sun wheel and the bore of the drivenshaft, information or media can be passed through the gear system,through the conduit formed by these bores. As a rule, this conduit isconcentric with the axis of rotation of the drive shaft and drivenshaft.

In a first advantageous feature of the invention, with a free endportion pointing toward the driven shaft, the drive shaft can beinserted into the bore in the driven shaft. Because of this provision,the possibility exists of guiding or supporting the drive shaft in thedriven shaft and optionally of providing further provisions, forinstance providing sealing in the overlapping region between the driveshaft and the driven shaft.

It also proves to be advantageous that the free end portion of the driveshaft is received by a bush seated in the bore in the driven shaft. Bymeans of a bush, an individual adaptation of the external dimensions ofthe drive shaft in the region of the free end portion and of theinternal dimensions of the driven shaft can be made.

Especially advantageously, a sealing system is disposed between thedrive shaft and the driven shaft, so that fluid media can be fed throughthe hollow shafts. In this respect, because of this provision, it ispossible to dispense with such additional provisions as hoses orpipelines, since the driven shaft and drive shaft that rotate relativeto one another are already sealed off from one another.

In order maximally to avoid the friction torques, which particularly inmicro gear systems are extremely unwanted, it is provided in anotheradvantageous feature of the invention that the sealing system isembodied as a contactless seal, preferably a labyrinth seal, andpreferably a plurality of annular grooves, disposed offset with respectto the longitudinal axis of the gear system, are disposed in the outerwall of the drive shaft or the inner wall of the bush or of the drivenshaft.

In a further advantageous feature of the invention, it is provided thatthe drive shaft and/or the driven shaft is supported in a housing of thegear system by means of rotary bearings, in particular preferablypre-loaded radial-groove ball bearings. By this provision as well, thefriction torques that occur in the gear system are reduced to a minimum.Also by means of this provision, the precision of concentricity of thedrive shaft and driven shaft is assured, so that the micro gear systemhas a high repeating accuracy, constant instantaneous transmissionproperties, and a long life expectancy.

In another advantageous refinement the planet wheels are embodied as aring which has spring elasticity in the radial direction. Because of theuse of elastic planet wheels, the tooth flank-backlash is eliminated,and a practically backlash-free gear system is created. Also by thisprovision, production inaccuracies, wear, and problems caused by soilingcan be compensated for.

A structurally simple design of the gear system is also obtained byproviding that the housing of the gear system comprises two housingparts, which are preferably embodied annularly, mesh with one another incuplike fashion, and between them receive the wave generator, sun andplanet wheels, and flexible, circular, and dynamic splines.

In another concept of the invention, the gear embodied as a micro gearsystem has a diameter of less than 10 mm and/or a height of ca. 1 mmand/or a transmission ratio of ca. 100 to ca. 1500 and/or a tooth moduleof less than 100 μm.

It also proves to be advantageous to produce such a micro gear system ofthe invention using the LIGA process, as described for instance inRadiat, Phys. und chemistry 45 (1995), No. 3, pages 349-365, W. Ehrfeld,H. Lehr, “Deep X-ray Lithography for the production of three-dimensionalMicro Structures from Metals, Polymers and Ceramics”. The LIGA processincludes the steps of lithography, electroforming and molding.

If the gear system of the invention is embodied as a strain wave gearsystem, it is attractive for the planet wheels to mesh with the internalteeth of an annular flexible spline.

It also proves advantageous that the flexible spline has external teeth,which mesh with the internal teeth of an annular circular spline andwith the internal teeth of an annular dynamic spline.

Advantageously, the circular spline and the dynamic spline are axiallyoffset from the longitudinal axis of the gear system.

In another feature of the invention, the flexible spline and circularspline in particular have a slightly different number of meshing teeth.

The dynamic spline is connected to the driven shaft in a manner fixedagainst relative rotation.

If the gear system is embodied as a revolving wheel gear system or aplanetary gear system with floating bearing of the planet wheels withouta carrier for guiding the planet wheels, or for torque pickup, that is,if it can be classified in the category of Wolfrom gear systems, thenthe planet wheels mesh with internal teeth of a split ring gear.Although it is already possible to use two planet wheels, it isattractive to use three planet wheels, which has the advantage that thesun wheel is supported by the planet wheels in a statically determinedway. Also the planet wheels can likewise be embodied as compliant in theradial direction, to compensate for inaccuracies caused by play, wear,and production variations.

Advantageously, the ring gear is split into one ring gear structurallyconnected to the housing and one driven ring gear, and the two ringgears may be axially offset from the longitudinal axis of the gearsystem.

Preferably, the ring gear structurally connected to the housing and thedriven ring gear have in particular a slightly different number of teethmeshing with the teeth of the planet wheels.

Advantageously, the driven ring gear is connected to the driven shaft,or an integral component of this driven shaft, in a manner fixed againstrelative rotation.

Finally, it proves advantageous that the planet wheels are supported ina floating fashion.

Further characteristics, advantages, possible applications andembodiments of the present invention will become apparent from theensuing description of an exemplary embodiment in conjunction with thedrawings. All the characteristics described and/or shown in the drawingsare the subject of the present invention either by themselves or inarbitrary appropriate combination, regardless of how they are recited inthe claims and regardless of the claims dependencies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one exemplary embodiment of a micro gear system of theinvention in a sectional view;

FIG. 2 shows a strain wave gear system of cup-like construction (FIG. 2a) and in flat construction (FIG. 2 b), each in a sectional view, in theprior art;

FIG. 3 is a schematic illustration of a Harmonic Drive gear system witha wave generator comprising a sun wheel and two planet wheels, toillustrate the mode of operation;

FIG. 4 is a schematic illustration to explain the mode of operation of aplanetary gear system with floating bearing of the planet wheels withouta carrier, for instance a Wolfrom gear system;

FIG. 5 shows the gear system of FIG. 4 in a sectional view; and

FIG. 6 shows one exemplary embodiment of a Wolfrom gear system of theinvention, again in a sectional view.

DETAILED DESCRIPTION OF THE INVENTION

The fundamental mode of operation of gear systems 10, in particularmicro gear systems, on the Harmonic Drive (registered trademark)principle of cup-type or flat construction has already been described indetail at the outset, so that no further description of FIGS. 2 and 3 isneeded. Moreover, in the exemplary embodiments of the prior art in FIGS.2 and 3, for designating identical components in the exemplaryembodiment of the present invention shown in FIG. 1, identical referencenumerals will be used, so that the gear systems of FIGS. 2 and 3 areself-explanatory, especially in conjunction with the prior art discussedat the outset.

The gear system 10 or micro gear system on the Harmonic Drive principleshown in FIG. 1 is of flat construction and has a wave generator 12,which comprises a planetary gear system, which in turn comprises one sunwheel 14 and at least two planet wheels 16 that mesh with the sun wheel14. The sun wheel 14 is embodied annularly in the exemplary embodimentof FIG. 1 and is seated in a manner fixed against relative rotation on adrive shaft 20 that is disposed centrally with respect to a longitudinalaxis 18 of the gear system.

Naturally, the possibility also exists that the sun wheel 14 is inparticular an integral component of this drive shaft 20.

The planet wheels 16 mesh with the internal teeth of an annular flexiblespline 22, which in turn has external teeth. The external teeth of theflexible spline 22 mesh both with the internal teeth of an annularcircular spline 24 and the internal teeth of an annular dynamic spline26. The circular spline 24 and the dynamic spline 26 are each annular inshape and are axially offset from one another relative to thelongitudinal axis of the gear system. The flexible spline 22 andcircular spline 24 have a slightly different number of intermeshingteeth; for instance, the difference in the number of teeth is two, whilethe dynamic spline 26 has the same number of teeth as the flexiblespline 22 and thus serves as a power takeoff element. The dynamic spline26 is connected to a driven shaft 28 in a manner fixed against relativerotation. In the present exemplary embodiment, as can be seen from FIG.1, the annular dynamic spline 26 is disposed in a receptacle, widened incuplike fashion, of the driven shaft 28 and rests with its outer wall onthe inner wall of the cuplike receptacle and is joined to the receptaclein a manner fixed against relative rotation, that is, rigidly. It shouldalso be noted that not only the drive shaft 20 but also the driven shaft28 are disposed essentially centrally with respect to the longitudinalaxis 18 of the gear system.

To make it possible for media or information to be passed through thisgear system 10, both the drive shaft 20 and driven shaft 28 are eachembodied as a hollow shaft 30, 32, with a respective continuous bore 34,36 or other conduit. Thus for micro gear systems, for the first time,the possibility is made available of transporting media, information orthe like through the center of rotation of the gear, so that a laserbeam, optical fibers, shafts, supply lines or even media can be carriedthrough the gear system directly on the central axis of rotation. It isthus unnecessary for information, media or the like to be guided aroundthe gear system, which would require major structural expense and alarge amount of installation space.

As can also be seen from FIG. 1, the drive shaft 20 protrudes, with afree end portion 38 pointing toward the driven shaft 28, into the bore36 or conduit in the driven shaft 28. The free end portion 38 of thedrive shaft 20, in the exemplary embodiment of FIG. 1, is also receivedby a bush 40 that is seated in the bore 36 or conduit in the drivenshaft 28. Between the drive shaft 20 and the driven shaft 28, a sealingsystem 42 is provided, which makes it possible, despite the relativemotion of the drive shaft 20 and driven shaft 28, to pass a mediumdirectly through the two hollow shafts 30, 32. Although conventionalsealing systems, such as rubber seals or the like, in which an elasticsealing face is pressed for sealing purposes between the components thatmove relative to one another can certainly be used, such conventionalseals, especially in view of micro systems, have the disadvantage ofcreating considerable additional frictional torque in the seal andwearing the sealing faces and thus making the seal gradually leak. Inthis respect, in the preferred exemplary embodiment of FIG. 1, thesealing system 42 is embodied as a contactless seal, preferably as alabyrinth seal 44, and preferably a plurality of annular grooves 46,offset relative to the longitudinal axis of the gear system, aredisposed in the outer wall of the drive shaft 20 or in the inner wall ofthe bush 40 or driven shaft 28.

The drive shaft 20 and/or driven shaft 28 are supported in a housing 48of the gear system, by means of rotary bearings 50, in particularpreferably pre-loaded radial-groove ball bearings. The planet wheels 16are also embodied as a ring, which has spring elasticity in the radialdirection. The housing 48 of the gear system 10 comprises two housingparts 52, 54, which are preferably embodied annularly, mesh with oneanother in cuplike fashion, and between them receive the wave generator12, the sun wheel 14 and planet wheels 16, and the flexible spline 22,circular spline 24, and dynamic spline 26.

The gear system 10 embodied as a micro gear system is distinguished by adiameter of less than 10 mm (preferably about 8 mm), a height of about 1mm, a transmission ratio of about 100 to about 1500 (preferably about500 to 1000), and a tooth module of less than 100 (30 to 40) μm,preferably 34 μm, and is preferably produced by the LIGA processincluding the steps of lithography, electroforming and molding.

The embodiment according to the invention of the drive shaft 20 and thedriven shaft 28 each as a hollow shaft 30, 32 with a continuous bore 34,36 or continuous conduit can also be realized in other revolving wheelgear systems or planetary gear systems with floating planet wheels.

In FIGS. 4 and 5, a so-called Wolfrom gear system 58 is shownschematically; in a simple embodiment, it is quite similar to thestructure of the strain wave gear system 56 described above. To thisextent, identical components are identified by the same referencenumerals. Below, only the differences in the structure of the gearsystem will therefore be addressed.

Essentially, in the Wolfrom gear system of the embodiment of theinvention shown in FIG. 6, the flexible spline 22 of the exemplaryembodiment of FIG. 1 is omitted. The Wolfrom gear system 58 has a splitring gear 64, which is formed of both a ring gear 60 structurallyconnected to the housing and a driven ring gear 62, and these ring gearsare axially offset from the longitudinal axis 18 of the gear system. Thering gear 60 structurally connected to the housing and the driven ringgear 62 have a slightly different number of teeth that mesh with theteeth of the planet wheels 16. The driven ring gear 62 is connected tothe driven shaft 28 in a manner fixed against relative rotation, and thepreferably three planet wheels 16 are floatingly supported.

If the sun wheel 14 is set into rotation via a rotational motion of thedrive shaft 20, then this motion is transmitted to the preferably threeplanet wheels 16, whose external teeth mesh with the internal teeth ofthe sun wheel 14. In the same way, the planet wheels 16, with theirexternal teeth, mesh with the internal teeth of the ring gear 64, or inother words both the ring gear 60 structurally connected to the housingand the driven ring gear 62. To this extent, the driven ring gear 62 isthen set, depending on the gear ratios that are to be set individually,into a rotation about the longitudinal axis 18 of the gear system, andthis rotation is correspondingly transmitted to the driven ring gear 62and driven shaft 28 because of the connection of this driven ring gearto this drive shaft in a manner fixed against relative rotation.

In this modified Wolfrom gear system 58 as well, there is a majoradvantage that both the drive shaft 20 and the driven shaft 28 areembodied as hollow shafts 30, 32 and have a continuous conduit, so thatinformation or fluid media, for instance, can be passed through the gearsystem 10.

1. A micro gear system on the principle of the strain wave gear system(56) or Wolfrom gear system (58) of flat construction, which has aplanetary gear system, comprising a sun wheel (14) and at least two ormore planet wheels (16) meshing with the sun wheel (14), in which thesun wheel (14) is seated in a manner fixed against relative rotation ona drive shaft (20) disposed centrally with respect to a longitudinalaxis (18) of the gear system or is a component of the drive shaft (20),and the planet wheels (16) mesh indirectly or directly with a gear wheelwhich is connected in a manner fixed against relative rotation to adriven shaft (28), which is disposed centrally with respect to thelongitudinal axis of the gear system, and the planetary gear system hasa tooth module of less than 100 μm, and the planet wheels (16) arefloatingly supported without a carrier and each planet wheel (16) isembodied as a ring which has spring elasticity in the radial direction,characterized in that the drive shaft (20) and driven shaft (28) areembodied as a hollow shaft (30, 32) and jointly provide a continuousbore (34, 36) or a continuous conduit which passes through a housing(48) of the gear system, the housing (48) of the gear system comprisestwo housing parts (52, 54), which mesh with one another and form thehousing in the shape of a cup, and the housing parts (52, 54) receivethe sun wheel (14) and the planet wheels (16) therebetween.
 2. The microgear system of claim 1, produced by a process including the steps oflithography, electroforming and molding.
 3. The micro gear system ofclaim 1, characterized in that the rotary bearings (50) are embodied asO-braced radial-groove ball bearings.
 4. The micro gear system of claim1, characterized in that the housing parts (52, 54) are embodied asannularly shaped elements.
 5. The micro gear system of claim 1,characterized in that the drive shaft (20) or driven shaft (28) issupported in the housing (48) of the gear system by rotary bearings(50).
 6. The micro gear system of claim 1, characterized in that thedriven shaft (28) has an inner end including an outwardly extendingannular flange which provides a cup-shaped receptacle, the sun wheel(14) is disposed in the receptacle, and the planet wheels (16) are heldbetween the flange and the housing part (52).
 7. The micro gear systemof claim 1, characterized in that the drive shaft (20) has a free endportion (38) which extends into the bore (36) in the driven shaft (28),and a sealing system (42) is disposed between the free end portion (38)of the drive shaft (20) and the driven shaft (28).
 8. The micro gearsystem of claim 7, characterized in that the free end portion (38) ofthe drive shaft (20) is received by a bush (40) seated in the bore (36)or conduit in the driven shaft (28).
 9. The micro gear system of claim8, characterized in that the sealing system (42) is embodied as acontactless seal comprising a labyrinth seal (44) including a pluralityof empty annular grooves (46), disposed offset with respect to thelongitudinal axis (18) of the gear system, and disposed in the outerwall of the drive shaft (20) or the inner wall of the bush (40).
 10. Themicro gear system of claim 7, characterized in that the sealing system(42) is disposed between an outer wall of the drive shaft (20) and aninner wall of the driven shaft (28).
 11. The micro gear system of claim10, characterized in that the sealing system (42) is embodied as acontactless seal comprising a labyrinth seal (44) including a pluralityof empty annular grooves (46), disposed offset with respect to thelongitudinal axis (18) of the gear system, and disposed in the outerwall of the drive shaft (20) or the inner wall of the driven shaft (28).12. The micro gear system of claim 1, characterized by a diameter ofless than 10 mm, a height of about 1 mm, a transmission ratio of about100 to about 1500, and a tooth module of 30 to 40 μm.
 13. The micro gearsystem of claim 9, characterized by a diameter of about 8 mm.
 14. Themicro gear system of claim 12, characterized by a tooth module of about34 μm.
 15. The micro gear system of claim 1, characterized in that thegear system (10) is embodied as a strain wave gear system (56), and theplanet wheels (16) mesh with internal teeth of an annular flexiblespline (22).
 16. The micro gear system of claim 15, characterized inthat the flexible spline (22) has external teeth, which mesh withinternal teeth of an annular circular spline (24) and with internalteeth of an annular dynamic spline (26).
 17. The micro gear system ofclaim 16, characterized in that the circular spline (24) and the dynamicspline (26) are axially offset from the longitudinal axis (18) of thegear system.
 18. The micro gear system of claim 16, characterized inthat the flexible spline (22) and circular spline (24) have a slightlydifferent number of teeth in the meshing teeth.
 19. The micro gearsystem of claim 16, characterized in that the dynamic spline (26) isconnected to the driven shaft (28) in a manner fixed against relativerotation.
 20. The micro gear system of claim 19, characterized in thatthe driven shaft (28) has an inner end including an outwardly extendingannular flange which provides a cup-shaped receptacle, the annulardynamic spline (26) is disposed in the receptacle, and the planet wheels(16) are held between the flange and the housing part (52).
 21. Themicro gear system of claim 1, characterized in that the gear system (10)is embodied as a Wolfrom gear system (58) including three planet wheels(16) which mesh with internal teeth of a split ring gear (64).
 22. Themicro gear system of claim 21, characterized in that the ring gear (64)has one ring gear (16) structurally connected to the housing and onedriven ring gear (62), which are axially offset from the longitudinalaxis (18) of the gear system.
 23. The micro gear system of claim 22,characterized in that the ring gear (60) structurally connected to thehousing and the driven ring gear (62) have a slightly different numberof teeth meshing with the teeth of the planet wheels (16).
 24. The microgear system of claim 22, characterized in that the driven ring gear (62)is connected to the driven shaft (28) in a manner fixed against relativerotation.
 25. The micro gear system of claim 24, characterized in thatthe driven shaft (28) has an inner end including an outwardly extendingannular flange which provides a cup-shaped receptacle, the driven ringgear (62) is disposed in the receptacle, and the planet wheels (16) areheld between the flange and the housing part (52).
 26. The micro gearsystem of claim 1, characterized in that a portion of the drive shaft(20) or the driven shaft (28) extends into the other of said shafts (20)and the driven shaft (28) for sealing the hollow shafts (30, 32) againsteach other to allow media to pass through the continuous bore (34, 36)without leakage.
 27. The micro gear system of claim 26, characterized inthat the sealing system (42) is embodied as a contactless sealcomprising a labyrinth seal (44) including a plurality of empty annulargrooves (46), disposed offset with respect to the longitudinal axis (18)of the gear system.